Cygwin: mbrtowi: define replacement for mbrtowc, returning UTF-32 value

Cygwin: mbrtowi: define replacement for mbrtowc, returning UTF-32 value

Given how UTF-16 isn't capable to hold all Unicode chars in a single
wchar_t, we need a function returning a wint_t value representing
a UTF-32 value for comparison functions.  Fortunately the important
wide character functions like towupper/towlower, isw<class>, iswctype,
etc, already take wint_t values and newlib handles them as UTF-32.

If only we had switched wchar_t to 32 bit way back when... sigh.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>